Tandem drive mechanism



NOV. 17, 1953 E F, NQRELlvUs 2,659,246

TANDEM DRIVE MECHANISM Filed March 25, 1949 2 Sheets-Sheet l NOV. 17, 1953 E, F, NOREUUS 2,659,246

TANDEM DRIVE MECHANISM Filed March 25, 1949 2 Sheets-Sheet 2 Patented Nov. 17, 19555 U N ITED STATES PATENT `TOI-'IFICE@ ""D'RIVE tolAisiii Emil FrNorelius, springfield, i111., assigner ...to Allis-Chalmers Manufacturingompany, :Mili Waukee, v Wis.-, a corporation of Delaware "Apiiucdttiriuai'ch'zs, 1949, :serial N, l'tenait-'1 l The invention relates to vehiclerunnin'g gear, and itis 'concerned more particularly with va mul'- ftiple wheel drive for v`motor vehicles;

Differential ineclianisms for transmittingip'ower 'fronti the engine to the propelling wheelsof -a fmo'- Vtor vehicle-are usually-employed in such amanner that tlf-1e entire 'dilieiitfah that is, th'eitwo half shaft gears and the spider on which the planet pinions are Yrounte'd rotate as la unit "during straight-ahead dni/eef 'the rimer vehicle, the drivingpower being usually applied to the spider,

-andthegear raue 'between che half-shaft gears,

at zero speed'of the fs'pider, being equal to unity iso that-there will be vrib relative rotation between these gears while thewheels are Vdriven 'at equal Vspeeds in 'th'e'same direction.'

While the use 4ci su'eh adiilerentialfmechanism is auniversally -a'dop'ted practice in motor vehicles havingthe usual two lpropelling wheels at vthe rear, vits Vuse ha'sfalso been proposed for tandem drive motorjvehiclesin which a pair of power driven tandem wheels is mounted at each side of the Vvehicle body. that case, as disclosed for instance in U. fs. eaten@ 1,713,133, grantee May 14, 1929, to -J Junkih for Multiple Wheel Drive, the engine poweris applied to the spider of a -rnain differential 1of the hereinabove -'mentioned character, and thehalf shafts of the main difiere jential are connected, respectively, with the spiders lo'ftwo auxiliary vdifferentials vofthe same character, t h'e half shaft gears of one of the auxiliary di-ierentials'being connected, respectively, with the tandem wheels-at one side vof the vehicle, and the half Vshaft gears fof the other auxiliary ditfer `ential being connected, respectively, with the tan- `tion to provide -anifmproved drive mechanism of the hereinabove stated character 'which will be operative to ydrive bothwhe'els simultaneously in such a manner `th'attlie rim speedjaiid fthe :rim 'pull -of one of `'the simultaneously driven :wheels will 'be equal, or substantially equal, 't the `rin lspeed andthe rim pul'lof `lthe other "'f the felini-1ltaheousiyuriveniwnns.

T24 claims. (ci. 74-7165 .be manufactured .at `relatively llow costs.

A 'further object -o'f the linventio'n is to `provide 'an improved differential and drive gearing for a pair of vehicle `whe'elspwhi'cli will permit differ."- ential rotation of `the wheels yandtransmit-rota Vtion of the l'driveshaftiat a reducedspeed lsimultaneously'toy both wheels. Y

A further bljectrof 'the invention is to provide van improved Adifferential and drive gearing Aofl the hereinabove statedfcharacter, in which the total gear .reduction between the drive shaft and both vwheels is twice as large as the gear reduction which 4'obtains V'between vthe 'drive lshaft andone of the wheels when .the other wheel islocked against rotation. i

rA further object of Vthe invention is to provide an improved .differential and drive gearing .for a pair of vehicle wheels which, in addition t0 ac'- complishing all of `the :foregoing objects, will fb'e relatively simple yand compact, and which may Briefly and specifically, the improved multiplewheel .drive mechanism, as contemplated by the invention, comprises an epicyclic gear train :of the type in which threerelatively rotatable power transmitting veleriierits forming a sun geana 'coun'- tergear, and aspideryrespectively, are operatively interconnected by planet pinions on the spider `so that'the gear ratio between said sun 'and countergears, at zero speed of the spider, is different from unity; and suitably constructed driving connections of 4said three power transmitting elements with a drive Shaft, and with one and the other of a pair of vehicle wheels, respectively. The epicyclic gear train is preferably kso constructed-'that the .gear rat-io between the sun fgear and the countergear, at Zero yspeed 'of the spider, is'larger than unity, and the gear vratio between the fdrive shaft and one of the wheels is preferably equal, or substantially equal, to the gear ratio between the drive lshaft and the 'otherfof the wheels.

These Aand other-'objects 'and advantages are attained yby the present invention, various `novel kfeatures of which will be apparent from the deynear side ofv the tractor being omitted for purposes of exposure; ,Y

Fig. `2 is a top Vview :oivalink'andfother rparts'lat thenear side of. the .'.tractor shown vin Figf -1 'portions '0f .the :lilik 'alldassocatediparts Fig.

being shown 'in section en iure ns-r1 of rig. 1.;

Fig. 3 is a sectional view, on line III-III in Fig. 1, of a tandem drive mechanism at the near side of the tractor shown in Fig. 1;

Fig. 4 is a diagram of the tandem drive mechanism shown in Fig. 3;

Figs. to 9 illustrate ve modifications, respectively, of the mechanism shown in Fig. 3, the showing of the parts in each of Figs. 5 to 9 being diagrammatic and analogous to that of Fig. 4.

Referring to Fig. 1, a pair of tandem wheels W1 and W2 at the far side of the tractor, and `a corresponding pair of tandem wheels at the`near side of the tractor, shown in Figs.V 2 and 3, Support a main body 3 in an elevated position above ing by controlling the drive to the traction devices,

at the opposite sides of the tractor body. For a fuller disclosure of a suitable power steering 'mechanism which may be enclosed in the rear the ground, the rear tandemwheels Wi'being rotatably mounted at the opposite ends of a trans- Y A verse rear axle housing 4, which forms part of the main body 3, and avtransverse leaf spring 6 being pivoted at its center on the main body 3 'and sustained at its opposite ends by the front tandem wheels W2. As shown in Fig. l, a support 'I for the near front tandem wheel W2 is connected with the rear axle housing 4 by means of yan upper link 8 and a lower link 3, and a corresponding support 1 (not shown) is similarly connected by means of an upper and a lower link Vto the rear axle housing 4 at the far side of the tractor for mounting the other front tandem Wheel W2, the transverse leaf spring 6 being suitably connected at its opposite ends in load transmitting relation with the wheel supports 1, respectively, at the opposite sides of the tractor.

Referring to Fig. 2, the upper link a is constructed in the form of a tapered frame having a relatively wide rear end and a narrow front end, ball and socket joints I I and I2 pivotally connecting the Wide rear end of the link 8 with a pair of transversely spaced upper lugs I3 and I4 on the rear axle housing 4, and a transverse pivot pin I6 connecting the narrow front end of the link 8 with a pair of upper lugs Il on the wheel support 1.

The lower link 9 is preferably constructed in -the form of a rod having a ball and socket connection I8 at its rear end with a lower lug I9 of the rear axle housing 4, and a ball and socket connection 2I with a lower lug on the wheel supthe same manner as the wheel support 'l at the near side of the tractor.

Referring to Fig. 3, an annular casing 22 which forms part of the rear axle housing 4, has a large axial end aperture, and a cover 23 for said aperture is rigidly secured to the casing 22 and has a central hub sleeve 24 which extends axially outward from the casing 22. The rear tandem wheel W1 is rotatably mounted on the hub sleeve 24 in axially fixed position by conical roller bearings 2B and 2l. A quill shaft 28 which extends axially through the hub sleeve 24 and which is rotatable relative to the latter and relative to the wheel W1, has a circumferential series of axially extendaxle housing-4 reference is here made to U. S.

f As shown in Fig. 3, a rotatable drive shaft S extends axially through the quill shaft 28, and

,the axially outer end of the drive shaft S is .mounted on the rear wheel Wi by means of a ball bearing 25 for rotation relative to the rear wheel and relative to the quill shaft 28. The mentioned Ypower steering mechanism, not shown, which is enclosed within the rear axle housing'4, is operable to control the transmission of engine power to the drive shaft S.

Integrally formed with the drive shaft S at the axially outer end of the latter is an externally toothed gear X which forms the sun gear of an epicyclic gear train within the rear wheel `W1. A disk Y which is rigidly secured at its outer periphery to the rim of the rear wheel W1 forms the spider of the mentioned epicyclic gear train, and a ring gear Z which is rigidly connected with the axially outer end of the quill shaft 28 by means vof a flange structure 32 forms a countergear of the mentioned epicyclic gear train. A circumferential series of studs 33 are secured to the spider Y, and spur gears P which form planet pinions of the mentioned epicyclic gear train are Lrotatably mounted on the studs 33, each planet Vshaft 34 is rotatably mounted on the casing 22 for rotation on` an axis at right angles to the ,axis of the rear wheel Wi, the line shaft 34 being rotatably supported in axially fixed position by means of a roller bearing 3B and a ball bearing 31 in an annular bearing support 38 which is rigidly secured to the casing 22 at the forward side of the latter. Integrally formed with the line shaft 34 is a bevel gear B which meshes with the bevel gear A within the casing 22.

The support 1 for the front wheel W2, as shown in Fig. 3, comprises a casing 4I and a cover 42 therefor which has a tubular central hub sleeve 43 on which the front wheel W2 is rotatably mounted in axially xed position by means of conical roller bearings 44 and 46. An axle shaft 4l for the front wheel W2 extends through the hub sleeve 43 and has a spline connection at its axially outer end with a disk 48 which is secured at its periphery to the rim of the front wheel W2.

Another line shaft 49 is rotatably mounted on the front Wheel support 'l for rotation on an axis at right angles to the axis of the front wheel AW2, the line shaft 49 being rotatably supported ing splines formed at its axially inner end. A the casing 4l at the rear side of the latter. mi

igiiyi-idd with timeline shaft 249 is a :bevel gear C whichnieshswith a' bevel gear tD on the axle shaft 41,'the .beveligear D .being splined on the axially inner end of the axle shaft 4l within the casing 4l and rotatably supported -by 'mea-ns o'sa double row ball .bearing 54 in a web 55 of the 4 casing 4 1, and by means ,of a roller bearing 56 in ile'cover 42. n V

A'n intermediate une .shaftsection51j-i's universally connected at 'its`opposi'te ends with the line shafts l34 and 149, respectively, a universal joint 58 connesting` the ,rearendof theintermediate line shaftlsection 51 with thefoward .end .of the line shaft 3l, and .aluniversal f-joint 1591eonnecting the forward endof the intermediateline shaft 'section'i51 with the Area-r Vend of the line shaft 49. riI-he forward end of the line shaft 34 has gfa-lcircumferential series of axially extending splines, iand'the 'universal joint 58 is mounted "on thesplined forward portion of -the Vline shaft Sulforriax'ial back andl forth 4 movement relative to thelatter. The universal joint 58,"on the other hand, fis Avmounted on the rear end of the line shaft-.4.93m axially xedposition. 'Ifheline shafts `3`4 and 45 together 'with ithe 'line shaft section YET-and universal joints :58 Vand59 form a shaft connection', bfetweenvthe axle,k housing 4 and the wnei supporti, 'for driving the from wheel W2, rand vwhich .shaft iconnection `is flexible ,and `extensibleso as' to accommodate up and down move- `neiii'f'o'f'thewh'eel supprt "l 'relative 'to the rear -f-.a,xlezhousing-4.A l

Itxwill:bezunderstoodthat .the tractorshown in r`Fig.V llhaszaztandempdrive mechanism atiits` far 'i sbotli drive shafts ,zsimultaneouslm ;n f conformity withiwellknownprinciples. y f

:Considering 'thegoperati'onr of: thev ,epicyclicgear Atrain :X,:Y, Z;andzof thebevel gear'train A,1B,

V;C,:D 'during propulsion tof the itractonqit' willf'be f noted that'ftheepicyclicsgear trainnotionly serves =as1azdiierential ,between the wheels `Wi 4and W2 .abut :also asrsa transmission g'earing :between 'the adrive'isliaftt's andthe wheels Wiand W2, whose igeai'.: ratioris ,differentfrom unity. v. .i As :shown 4in i Fig.:f3 ,a the :fepicyolie lgearrtrain X, Y, Zf is, so proxportioned V that ".tlie gear ratio between the vsun egearzXz and'Lthe-countergearv Z, atl zero'speed Vof #/'the spider i:Y,'. isalargeiw than funityfand :the bevel zgearftrainSA,:B,'. C, D,:fis=.so proportioned that' the iigearfratiobetweenithe bevlgears' Aiand D is sub- 'zzstantialiyaequal to'xthe gear ratiowhichobtains between .the-.countergeari Z1 and 'fthe'zspider'Y fat Vzero speediofithesunfgearfX. The rim diameter .,of-:the'zwheelWi:is equal to' the rim diameter lof iathe :wheel` W2,A andiduring Y.pri'ipulsion of theftrac- Attor ionic leverfground: theA rimv A:speed and* therim 'aWhen the wheels W1 and `W2^are 'thus' driven .at f Y thessamespeedsuch speed lisi-substantiallylower rthanirthe' rotary-speedf-of ...the drive'l 'shaft S, `the total reduction ratio between the.fd-riverfshaftil S and the wheels W1 and W2 being twice as large "as the reductioniratio"which'lobtains between the iadrivo shaft and 1one ofthe wheels when'theother lrieeleisloeked against rotation. 1 Infthe mecha- `.,nlslm as shown "Fig..f:`3,-rthe reduction ratio be- `rims/,eenstlief'drive ffshaftStand;.thefiwheel` Wifi at -Y, the sun gear;,Xvjsriggllyssecured to Whichobtain hetw n` duction ratio 'betveeiithe-.sun ,ge'ar'fx aan spider Y, and the totalfieduetionfzstaitio bie the drive shaft S and both wheels Wi and W2 is twice the reduction vratiobetween the sun gear X andthespider p For rurpses: 0f fr-'1211er eiplasaiionii ence will be made hereinbelow to Qthe tooth y in bers of the bevel gears A, B, C and D, and tothe tooth numbers `of the -sun `fg'earX, countergear z and any of the pianet p iiiipns AVP by thawinbols Ta, Tb, Tc, Td, TX, Tp'and TZ;- respectively. `Further, the symbol B. :two subletters will be used as follows:

'Red to designate ithe `'ratio lof the -spee'd aofy zthe "bevel gearv A lto'the speedfrof thelibevel .gear D. that-is:

Rzy to designate thereto ef rthe -eouiitefgeer speed to the spiderfspeedatzo speed of the sun gear X, that is:

Rex to designate thematic of the eountergear speed to the sun gear speed at zer'o speed of the spider Y, that is: A

#to-thespiderfspeed at zerozspeed of-thei'couiiter- Ryz, Rxz and Ryx to designate the inverse-values .of the ratios RzygiRZX 'and:Rxy,vrespectively.

p ,Rm/1 Rw1 andy Rm, to; designate thergear, ratios p, sungear-X spide Y and'contergear, tively,(anv ar tandem wheeVWi, Yat"'ze1'"o A`sp"eed.'; f the front tandem wh'eellW-z; v

Rime, Ryw2 and Rzw2 to designategthe gear`y ratios `which obtain between the "sunngearXftlie' spider `Yand the countergear Z,''ispectively, and the front tandemfwheeW2at;aeroesneedeofithearear -.tandem:ii/.heel#Wi,r ,Y l

Rm-fandswsmdess' at@isespectvslyitliefsear :ratio between fthe' drive :shaft arid 'ith Hyreal ywheel W1-atfzelzol speed-1M fthe, Y ,ntwhel :We f andthe searfratiovbetween, the :,drivefshaft iS the front wheel Wzgat zerofspeed ,ofigtliefrear .wheel W1.

k'The mechanism^`as *shown* in Fig. 3 and as 'diagrammatically ifshown: inFig. 14, fis soi-felonfStructed that:

I Radi-T2012 Infiiiemeciiiicatieng ir-nieiiieiiaiif" iii-assais- .,:1,lieft-S 1s connected in driving relation With-theifspider u V Y the rear wheel' Wi andthe countergear Z is connected rthrough: theabeveligeai. traina-A; vB;:'{Jfelwitlirl:the

7 front wheelV W2. The modified mechanism of Fig. is so constructed that:

Rzwl: 01 Rzw, (5)

Since the wheel W1 is rigid with the sun gear X, and the bevel gear A is rigid with the ring gear Z, it follows that:

`and from Equation 5:

Rua: or -Rze (7) Since the rear wheel W1 is rigid with the spider Y, and the bevel gear A is rigid with the sun gear X, it follows that:

RzwlY-Rxy and Raub: 01' @Rnd and from the Equation 8:

In the modification of the mechanism as shown in Fig. 7, the driving power is applied to the spider Y, the ring gear Z is rigidly connected with the rear wheel W1, and the sun gear X is connected with the front wheel W2 through the bevel gear train A, B, C, D. The modified mechanism as shown in Fig. '7, is so constructed that:

Rad: 01 @Rrr Rzwl: 01' wRmw, (3)

Since the ring gear Z is rigidly connected with the rear wheel W1, and the bevel gear A is rigid with the sun gear X, it follows that:

RxwlRxz and Rzw2=Rad and from Equation 8:'V

Rnd: O1 @R12 (10) In the modified mechanism as shown in Fig. 8, the driving power is applied to the sun gear X, the ring gear Z is rigidly connected with the rear wheel W1, and the spider Y is connected with the front wheel W2 through the bevel gear train A, B, C, D. The modied mechanism shown in Fig. 8 is so constructedl that:

Rywl: 0I @Rywa (11) Since the ring gear Z is rigidly connected with the rear wheel W1, and the bevel gear A is rigid with the spider Y, it follows that:

Ryw1=Ryz and RywgZRad and from Equation l1:

Rad: 01 RsRaiz (12) In the modied mechanism as shown in Fig. 9, the driving power is applied to the countergear Z, the sun gear X is rigidly connected with the rear wheel W1, and the spider Y is connected with the front wheel W2 through the bevel gear train A, B, C, D. The mechanism shown in Fig. 9 is so constructed that:

LSincethe sun gear. X is rigidly connected with 8 the rear 'wheel VW1,-and the bevel gear- A- is rigid with the spider Y, it follows that:

RgwlZRyx and Ryw2=Rad and from Equation 11:

As a practical example, the epicyclic gear train in each of Figs. 4 to 9 may be assumed to have the following tooth numbers:

17..:15; 13:33; 772:81 With these assumptions. representative gear data for the mechanisms shown in Figs. 4 to 9 may be tabulated as follows:

It will be noted that each of the mechanisms shown in Figs. 4 to 9, inclusive, incorporates an epicyclic gear train of the type in which three relatively rotatable power transmitting elements forming a sun gear X, a countergear Z and a spider Y, respectively, are operatively interconnected by planet pinion means on the spider and in which said sun gear, countergear and planet pinion means are constructed so that the gear ratio between a rst and a second of said power transmitting elements, at zero speed of the third of said power transmitting elements, is different from unity, as shown in the hereinabove tabulated ratio equations for the epicyclic gear train; that in each of the mechanisms shown in Figs. 4 to 9 the drive shaft S is connected in driving relation with said third power transmitting element; that each of the mechanisms includes a driving connection, independent of said rst and l third power transmitting elements, between said second power transmitting element and one of the wheels W1, W2, and that each of the mechanisms includes a fixed ratio gearing A, B, C, D connecting said rst power transmitting element in speed changing driving relation with the other of the wheels W1, W2. It will further be noted that in each of the mechanisms shown in Figs. 4 to 9, inclusive, the epicyclic gear train, the mentioned driving connection, and the iixed ratio gearing A, B, C, D are so constructed that the gear ratio between said third power transmitting element and said one vehicle wheel is equal or substantially equal to the gear ratio between said third power transmitting element and said other vehicle wheeL'that is, each'of the mechanisms shown in Figs. 4 to 9, inclusive, meets the following condition:

Remi: 01' R1 Rswg (14:)

It will further be noted that in each of the mechanisms shown in Figs. 4 to 9, the second mentioned power transmitting element of the epicyclic gear train X, Y, Z is rigidly secured 4to the wheel W1 and therefore the transmission ratio interconnected by planet pinion means on saidspider, a rotatable drive shaft'connected in driving relation with one of said sun and countergears, a driven element connected for rotation 1n unison with the other of said sun and countergears, another driven element connected for rotation in unison with said spider; a driving connection independent of said epicyclic gear train, between one of said driven elements and one of said vehicle wheels, and fixed ratio gearing independent of said epicyclic gear train, ccnnecting the other of said driven elements in speed changing driving relation with the other of said vehicle wheels, said driving connection and fixed ratio gearing being constructed and said sun gear, countergear and planet pinion means being proportioned so that the gear ratio which exists between said drive shaft and said one vehicle wheel at zero speed of said other vehicle wheel is equal or substantially equal to the gear ratio which exists between said drive shaft andrsaid other vehicle wheel at zero speed of said one vehicle wheel.

10. A differential and drive gearing as set forth in claim 9, in which the gears `of said epicyclic gear train are so constructed that the gear ratio between said sun and countergears, at zero speed of said spider, is larger than unity. 11. A differential and drive gearing for a pair of vehicle wheels comprising, in combination, an epicyclic gear train of the type in which three relatively rotatable power transmitting elements forming 'a sun gear, a countergear and a spider, respectively, are operatively interconnected by planet pinion means on said spider and in which said sun gear, countergear andV planet pinion means are constructed so that upon application of a predetermined driving speed in a predetermined direction to said sun gear, at zero speed of said spider, said countergear will be driven at a slower speed in the opposite direction, a nonreversing driving `connection between a rst of said power transmitting elements and one of said vehicle wheels, and xed ratio, speed changing, reverse gearing operatively interposed between asecond of said power transmitting elementsand the other of said vehicle wheels, and a rotatable drive shaft connected in drivingrelation with the third of said power transmitting elements, said driving connectionY and said fixed ratio gearing being so constructed, and said sun gear, countergear and planet pinion means being so proportioned thatthe gear ratio which exists between said third power transmitting element and said one vehicle wheel, at zeroV speed ofthe other of said vehicle wheels, is equal or substantially equal to the gear ratio which exists between' said third power transmitting element and said other vehicle wheel at zero speed of said one vehicle wheel. 12. A differential and drive gearing as set forth in' claim 11, in which said rotatable drive shaft is connected in driving relation with said sun gear. 13. A differential and drive gearing Vas set forth in claim 11, in which said rotatable drive shaft isconnected in driving relation with said countergear.

' 14. A differential and Ydrive gearing as set forth in claim l1, in which said rotatable drive shaft is connected in driving relation with said sun gear, and inY which said reverse gearing is operatively interposed between said countergear and said other vehicle wheel. 15. A differential and drive gearing as set forth in claim 11, in which said rotatable drive shaft is connected in driving relation with said sun gear;

and in' which said reverse gearing is operatively interposed between said spider and said other vehicle wheel.

16. A differential and drive gearing as set forth in claim 1l, in which said rotatable drive shaft is connected in driving relation with said countergear', and in which said reverse gearing is operatively interposed between said spider and said other vehicle wheel.

l'l. A tandem drive mechanism comprising, in combination, a support, a pair of vehicle wheels operatively mounted on said support in tandem relation to each other, a quill shaft mounted on said supportcoaxially with one of said wheels for rotation relative to the latter and relative to said support, a rotatable drive shaft extending through said quill shaft, a drive gear secured to said drive shaft in ncnrotatable relation thereto, planetary pinion means in mesh with said drive gear, power transmitting means comprising a rotatable ring gear coaxial with said drive shaft and in mesh with said planetary pinion means, other power transmitting means comprising a support for said planetary pinion means rotatable about the axis of said drive shaft, one of said power transmitting means being secured to said one wheel for rotation `in unison therewith, 'and the other of said power transmitting means being secured to said quill shaft for rotation in unison therewith, and a speed changing reverse driving connection between said quill shaft and the other of said wheels, affording a gear ratio between said quill shaft and said other Wheel equal, or substantially equal, to the gear ratio which exists between said quill shaft and said one vehicle wheel at zero speed of said drive shaft.

' 1-8. A tandem drive mechanism comprising, in combination, a support, a pair of vehicle wheels operatively mounted on said support in tandem relation to each other, a quill shaft mounted on said support coaxially with one of said wheels, for rotation relative to the latter and relative to said support, a rotatable drive shaft extending through said quill shaft, an externally toothed drive gear secured to said drive shaft, an internally toothed ringgear secured to Vsaid quill shaft, planetary pinion Y means rotatably mounted on said one wheel land in meshing relation with said drive and ringY gears, and a speed reducing reverse driving connection between said quill shaft and the other of said wheels, affording a gear ratio between saidquill shaft and said other vehicle wheel equal, or substantially equal, to the gear ratio which exists between said quill shaft and said one vehicle wheel at zero speed of said drive shaft.

19. A tandem drive mechanism comprising, in combination, a support, a pair of vehicle wheels mounted on said support for rotation on axes, respectively, spaced transversely from each other, an epicyclic gear train of the type in which three relatively rotatable power transmitting elements forming a sun gear, a countergear and a. spider, respectively, are operatively interconnected by planet pinion means on said spider, and in which said sun gear, countergear and planet pinion means are constructed so that the gear ratio between said sun and countergears, at zero speed of said spider, is larger than unity, a quill shaft rotatably mounted on said support in coaxial and rotatively loose relation to one of said vehicle wheels and secured for rotation in unison with a first of said power transmitting elements, means securing a second of said power transmitting elements for rotation'in unison with said one vehicle wheel, a drive shaft extending through and rotatable relative to said quill shaft and nonrotatably secured to the third of said power transmitting elements; a first pair of intermeshing gears, one nonrotatably and coaxially secured to said quill shaft, and the other mounted on said support for rotation on an axis extending angularly relative to said wheel axes; a second pair of intermeshing gears, one nonrotatably and coaxially secured to the other of said vehicle wheels, and the other mounted on said support for rotation on an axis extending angularly relative to said wheel axes; and shafting connected in driven relation with said other gear of said iirst pair and in driving relation with said other gear of said second pair; said first and second pairs of intermeshing gears being constructed so that the gear ratio between said one gear of said rst pair and said one gear of said second pair is equal or substantially equal to the gear ratio which exists between said first and second power transmitting elements of said epicyclic gear train at zero speed of the third of said power transmitting elements.

20. A tandem drive mechanism as set forth in claim 19, in which said countergear, spider and sun gear are nonrotatably secured, respectively, to said quill shaft, one vehicle wheel, and drive shaft, and in which said rst and second pairs of intermeshing gears are constructed to afford a reversing driving connection between said quill shaft and said other vehicle wheel.

21. A tandem drive mechanism as set forth in claim 19 in which said countergear, spider and sun gear are nonrotatably secured, respectively, to said drive shaft, one vehicle wheel, and quill shaft, and in which said rst and second pairs of intermeshing gears are constructed to afford a reversing drive connection between said quill shaft and said other vehicle Wheel.

22. A tandem drive mechanism as set forth in claim 19, in which said countergear, spider and sun gear are nonrotatably secured, respectively, to said one vehicle wheel, drive shaft and quill 14 shaft, and in which said first and second pairs of intermeshing gears are constructed to aord a nonreversing driving connection between said quill shaft and said other vehicle wheel.

23. A tandem drive mechanism as set forth in claim 19, in which said countergear, spider and sun gear are nonrotatably secured, respectively, to said one vehicle WheeLquill shaft and drive shaft, and in which said first and second pairs of intermeshing gears are constructed to aiord a reversing driving connection between said quill shaft and said other vehicleiwheel.

24. A tandem drive mechanism as set forth in claim 19, in which said countergear, spider and sun gear are nonrotatably secured, respectively, to said drive shaft, quill and one vehicle wheel, and in which said rst and second pairs of intermeshing gears are constructed to afford a reversing driving connection between said quill shaft and said other vehicle wheel` n EMIL F. NORELIUS.

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